Rainbow Electronics MAX8720 - Manual (Page 23)

Suspend Transition (Pulse-Skipping
Operation Selected)
If the MAX8720 is configured for pulse-skipping opera-
tion (SKIP = GND) when SUS goes high, the MAX8720
immediately enters forced-PWM mode, ramping the
output voltage down to the S0, S1 programmed voltage
at the slew rate determined by R
TIME . The controller
blanks PGOOD (forced high impedance) until the tran-
sition is completed plus 8 extra R
TIME clocks—the
internal target voltage equals the selected S0, S1 DAC
voltage. After this blanking time expires, the controller
enters pulse-skipping operation.
When exiting suspend mode (SUS pulled low), the
MAX8720 immediately enters forced-PWM mode and
ramps the output up at the slew rate set by R
TIME . The
controller blanks PGOOD (forced high impedance) until
the transition is completed plus 8 extra R
TIME clocks—
the internal target voltage equals the selected D0–D5
DAC voltage. After this blanking time expires, the con-
troller returns to pulse-skipping operation.
Output Overvoltage Protection
The overvoltage-protection (OVP) circuit is designed to
protect the CPU against a shorted high-side MOSFET
by drawing high current and blowing the battery fuse.
The output voltage is continuously monitored for over-
voltage. If the output is more than 2.25V, OVP is trig-
gered and the circuit shuts down. The DL low-side
gate-driver output is then latched high until SHDN is
toggled or V
CC power is cycled below 1V. This action
turns on the synchronous-rectifier MOSFET with 100%
duty and, in turn, rapidly discharges the output filter
capacitor and forces the output to ground. If the condi-
tion that caused the overvoltage (such as a shorted
high-side MOSFET) persists, the battery fuse blows. DL
is also kept high continuously in shutdown when V
CC is
above the UVLO threshold.
Output Undervoltage Shutdown
The output UVP function is similar to foldback current
limiting, but employs a timer rather than a variable cur-
rent limit. If the MAX8720 output voltage is under 70%
of the nominal value, the PWM is latched off and won’t
restart until V
CC power is cycled or SHDN is toggled.
To allow startup, UVP is ignored until the internal DAC
reaches the final target plus 8 extra R
TIME
clocks.
UVP can be defeated through the no-fault test mode
(see the No-Fault Test Mode section).
No-Fault Test Mode
The over/undervoltage-protection features can compli-
cate the process of debugging prototype breadboards
since there are (at most) a few milliseconds in which to
determine what went wrong. Therefore, a test mode is
provided to disable the OVP, UVP, and thermal-shut-
down features, and clear the fault latch if it has been
set. The no-fault test mode is entered by forcing 12V to
15V on SHDN.
Design Procedure
Firmly establish the input voltage range and maximum
load current before choosing a switching frequency
and inductor operating point (ripple-current ratio). The
primary design trade-off lies in choosing a good switch-
ing frequency and inductor operating point, and the fol-
lowing four factors dictate the rest of the design:
• Input Voltage Range. The maximum value
(V IN(MAX) ) must accommodate the worst-case, high
AC-adapter voltage. The minimum value (V IN(MIN) )
must account for the lowest battery voltage after
drops due to connectors, fuses, and battery selector
switches. If there is a choice at all, lower input volt-
ages result in better efficiency.
• Maximum Load Current. There are two values to
consider. The peak load current (I LOAD(MAX) ) deter-
mines the instantaneous component stresses and fil-
tering requirements and thus drives output-capacitor
selection, inductor saturation rating, and the design of
the current-limit circuit. The continuous load current
(I
LOAD
) determines the thermal stresses and thus dri-
ves the selection of input capacitors, MOSFETs, and
other critical heat-contributing components.
• Switching Frequency. This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage, due to MOSFET switching losses that
are proportional to frequency and V
IN
2 . The opti-
mum frequency is also a moving target, due to rapid
improvements in MOSFET technology that are mak-
ing higher frequencies more practical.
• Inductor Operating Point. This choice provides
trade-offs between size vs. efficiency, and transient
response vs. output ripple. Low inductor values pro-
vide better transient response and smaller physical
size, but also result in lower efficiency and higher
output ripple due to increased ripple currents. The
minimum practical inductor value is one that causes
the circuit to operate at the edge of critical conduc-
tion (where the inductor current just touches zero
with every cycle at maximum load). Inductor values
lower than this grant no further size-reduction benefit.
The optimum operating point is usually found
between 20% and 50% ripple current. When pulse
skipping (SKIP low and light loads), the inductor
MAX8720
Dynamically Adjustable 6-Bit VID
Step-Down Controller
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